The high molecular weight form of the Sinorhizobium meliloti 1021 symbiotic exopolysaccharide succinoglycan is sufficient to mediate invasion of host plants
K. JONES (1), H. Mendis (1), T. Madzima (1), C. Queiroux (1) (1) Florida State University, U.S.A.

The acidic exopolysaccharide succinoglycan produced by Sinorhizobium meliloti 1021 is required for this bacterium to invade the host plant Medicago truncatula and establish a nitrogen-fixing symbiosis. S. meliloti mutants that cannot make succinoglycan cannot initiate invasion structures called infection threads in root hairs. S. meliloti exoH mutants that cannot succinylate succinoglycan are also unable to form infection threads, despite the fact that they make large quantities of succinoglycan. Succinoglycan produced by exoH mutants cannot be cleaved by the glycanases encoded by exoK and exsH, and thus, high molecular weight (HMW) succinoglycan is produced by exoH mutants. One interpretation of the symbiotic defect of exoH mutants is that low molecular weight (LMW) succinoglycan is required for infection thread formation. However, our data show that production of the HMW form of succinoglycan by S. meliloti is sufficient for invasion of M. truncatula and that the LMW form is not required. We have determined that S. meliloti 1021 strains deficient in both the exoK- and exsH-encoded glycanases make HMW succinoglycan and no LMW succinoglycan. This demonstrates that glycanase cleavage is the route for production of the LMW form of succinoglycan in S. meliloti 1021. These double-glycanase mutants successfully invade M. truncatula roots and establish a productive symbiosis. Thus, the small, diffusible LMW form of succinoglycan is not required for its function.

Abstract Number: P2-30
Session Type: Poster